It is well known that conventional hydrocarbon surfactants can lower the surface tension of aqueous solutions to as low as 23 dynes/cm, while fluorinated surfactants can attain surface tensions in the 15 to 20 dynes/cm range. While fluorinated surfactants have proven to be much more efficient as surface tension depressants than hydrocarbon surfactants, their use has has been severely limited because of their high cost. The problem of attaining lowest possible surface tension with the smallest possible amount of highly priced fluorinated surfactants or surfactant/synergist systems has been the subject of many patents and publications. It is an object of the present invention to provide novel perfluoroalkyl or R.sub.f R.sub.f ion pair complexes which are capable of reducing the surface tension of aqueous systems to levels which are below the surface tensions achieved with equal amounts of the individual anionic and cationic perfluoroalkyl surfactants from which the novel R.sub.f R.sub.f ion-pair complexes are derived.
Four general classes of surfactants are commonly represented as useful surface tension depressants, namely cationic, anionic, amphoteric and nonionic surfactants. In many applications, combinations of hydrocarbon surfactants are used in order to achieve certain results. It is generally known that nonionic and amphoteric surfactants can be used in combination with each other as well as in combination with either anionic surfactants or cationic surfactants. Such surfactant mixtures are said to be compatible. It is also generally known that anionic and cationic surfactants should not be used in combination with each other, because they are incompatible. The reason combinations of anionic and cationic are called incompatible is that they react with each other and form poorly soluble hydrocarbon-hydrocarbon complexes plus salts, as described in Kirk-Othmer, Enclyclopedia of Chemical Technology, 19, 555, 2nd Ed., (1966) and Milton J. Rosen, Surfactants and Interfacial Phenomena, J. Wiley, 24.
These hydrocarbon cation/hydrocarbon anion complexes are simply referred to as R.sub.h R.sub.h complexes and have been described by Hummel as electroneutral substances. See Hummel, D., Identification and Analysis of Surface-Active Agents, Interscience, New York 1962 (p. 23). It has been reported that certain mixtures of hydrocarbon type anionic and cationic surfactants in some cases can exhibit low surface tensions. Corkill, see J. M. Corkill et al., Proc. Roy. Soc. (London) Series A273 (1963), was the first to show that mixtures of sodium decyl sulfate and decyltrimethylammonium bromide or the ion-pair hydrocarbon salt decyltrimethylammonium decyl sulfate give virtually identical surface tension-concentration curves and surface tension values as low as 22 dynes/cm.
Mannheimer (U.S. Pat. No. 3,661,945) showed that select structural types of anionics and cationics could give useful reaction products.
L. G. Anello and R. F. Sweeney (U.S. Pat. No. 3,839,343) has reported the preparation of a symmetrical polyfluoroisoalkoxyalkyl quaternary ammonium sulfates-[(CF.sub.3).sub.2 CFO(CF.sub.2).sub.s (CH.sub.2).sub.t NR.sub.x ].sup..sym. OSO.sub.2 O(CH.sub.2).sub.t (CF.sub.2).sub.s OCF(CF.sub.3).sub.2.
It has now been surprisingly found that R.sub.f R.sub.f ion-pair complexes made by reacting equimolar amounts of specific classes of anionic and cationic R.sub.f -surfactants are extremely useful and efficient surface tension depressants even though such R.sub.f R.sub.f complexes have been found to be very much less soluble than the anionic and cationic surfactants from which they were derived, and are in most practically insoluble in water (solubility of less than 0.001% by weight).
Most importantly, it was found that stable dispersions of the water-insoluble R.sub.f R.sub.f ion-pair complexes can be made and, quite significantly, it was found that such R.sub.f R.sub.f ion-pair complex dispersions provide considerably lower surface tension properties in water at extremely low concentrations than did equal amounts of either the anionic or cationic fluorochemical surfactants from which the R.sub.f R.sub.f ion-pair complex was derived.
Furthermore, it was found that the novel R.sub.f R.sub.f ion-pair complexes are most useful as additives to so-called AFFF agents or aqueous film forming foams. These so-called AFFF agents act in two ways:
(a) As aqueous foams they are used as primary fire extinguishing agents, and PA1 (b) As vapor sealants they prevent the reignition of fuels and solvents. PA1 SC=spreading coefficient PA1 .delta.a=surface tension of the lower liquid phase PA1 .delta.b=surface tension of the upper aqueous phase PA1 .delta.i=interfacial tension between the aqueous upper phase and lower liquid phase PA1 R.sub.f is perfluoroalkyl of 4 to 12 carbon atoms: PA1 A and A' independently represent a divalent covalent linking group of the formula ##STR2## wherein G, G' and G" independently represent --O--, --S--, --SO.sub.2 --, --SO.sub.2 NH--, --CONH--, ##STR3## n.sub.1 is 0 or 1; n.sub.2 and n.sub.3 are independently 0, 1 or 2; PA1 alkylene is straight or branched chain alkylene of 1 to 8 carbon atoms, with the proviso that each aliphatic chain A and A' contains no more than 18 carbon atoms and the R.sub.f and R.sub.f ' group, respectively, is bonded to the left hand side of said covalent linking group, PA1 and A additionally represents a direct bond; PA1 R.sub.1, R.sub.2 and R.sub.3 are lower alkyl; and PA1 Q is carboxylate, sulfonate, phosphate or phosphonate. PA1 R.sub.f is perfluoroalkyl of 4 to 12 carbon atoms; PA1 A and A' independently represent PA1 --CH.sub.2 CH.sub.2 --S--alkylene--G"--alkylene-- wherein G" is --SO.sub.2 NH-- or ##STR4## and each alkylene is straight or branched chain of from 1 to 6 carbon atoms; PA1 R.sub.1, R.sub.2 and R.sub.3 are methyl; and PA1 Q is carboxylate, sulfonate, phosphate or phosphonate. PA1 Q is sulfonate, carboxylate or phosphate. PA1 D. 0.05 to 0.3% by weight of an R.sub.f.R.sub.f complex. PA1 E. 0 to 30% by weight of fluorochemical surfactants. PA1 F. 0.1 to 0.3% by weight of fluorochemical synergist. PA1 G. 0.05 to 3% by weight of hydrocarbon surfactant. PA1 H. 0 to 25% by weight of solvent. PA1 I. 0 to 2% by weight of electrolyte. PA1 K. 0 to 2% by weight of foam stabilizer. PA1 L. 0 to 5% by weight of polysaccharide. PA1 M. Water in the amount to make up the balance of 100%.
It is this second property which makes AFFF agents far superior to other known fire fighting agents for fighting fuel and solvent fires. This vapor sealing action of an AFFF agent is achieved by the spreading of the aqueous AFFF agent solution over the fuel surface.
The criterion necessary to attain spontaneous spreading of two immiscible phases has been taught by Harkins et al., J. Am. Chem. 44, 2665 (1922). The measure of the tendency for spontaneous spreading is defined by the spreading coefficient (SC) as follows: EQU SC=.delta.a-.delta.b-.delta.i
where
If the SC is positive, the solution should spread and film formation should occur. The greater the SC, the greater the spreading tendency. This requires the lowest possible aqueous surface tension and lowest interfacial tension.
Based on the Harkins equation it is obvious that the most efficient surface tension depressants and interfacial tension depressants will yield aqueous film forming foams with the highest spreading coefficient.
Aqueous solutions with very low surface tensions used for the extinguishment of hydrocarbon fuel fires were first disclosed by N. O. Brace in U.S. Pat. No. 3,047,619 and 3,091,614. Brace utilized sprays of aqueous solutions containing R.sub.f surfactants of the beta-hydroperfluoroalkyl type. Similarly, Tuve, et al., disclosed the use of aqueous solutions containing R.sub.f -surfactants derived from perfluorinated acids of the R.sub.f COOH and R.sub.f SO.sub.3 H type as fire fighting foams in U.S. Pat. No. 3,258,423.
While R.sub.f -surfactants reduce the surface tension of aqueous solutions to as low as 15 dynes/cm, they generally do not reduce interfacial tension properties to the same degree as many hydrocarbon surfactants.
Aqueous solutions containing R.sub.f -surfactants and hydrocarbon surfactants having very low surface tensions as well as low interfacial tension properties, and therefore a positive spreading coefficient when measured against hydrocarbon solvents, were prepared by Klevens and Raison, J. Chim. Phys., 51, p. 1-8 (1959). The 3M Company, a manufacturer of R.sub.f -surfactants, recommended in their technical bulletins (3M Brand Fluorochemical Surfactants, June 15, 1963, pp. 1-45) the combination of R.sub.f -surfactants and hydrocarbon surfactants to achieve low surface tension and low interfacial tension in aqueous solutions and therefore a positive spreading coefficient.
Solutions of fluorochemical surfactants and hydrocarbon surfactants were used the first time by Ratzer as fire fighting foams and disclosed at the "Fourth Quinquennial Symposium on Fire Fighting Foam", Aug. 11/12/13, 1964 at Campobello, and published in the minutes of the above symposium, and in "Foam" October 1964, No. 24 (a publication of the Mearl Corporation, Ossining, N.Y.). Drs. Shinoda and Fujihira reported the use of mixtures of fluorochemical and hydrocarbon surfactants in the context of various commercial applications including fire extinguishing agents at a Meeting on the Research on Oil sponsored by the Japan Oil Chemists' Society and the Chemical Society of Japan Chemistry on Nov. 2-3, 1966 at Nagoya, Japan. Fire fighting agents based on aqueous solutions containing fluorochemical surfactants and hydrocarbon surfactants, as disclosed the first time by Ratzer are today commonly known as AFFF agent or Agent Film Forming Foams. Many patents disclosing AFFF agent compositions based on fluorochemical surfactants and hydrocarbon surfactants have since issued, such as U.S. Pat. Nos. 3,047,619; 3,257,407; 3,258,423; 3,562,156; 3,621,059; 3,655,555; 3,661,776; 3,677,347; 3,759,981; 3,772,195; 3,798,265; 3,828,085; 3,839,425; 3,849,315; 3,941,705; 3,952,075; 3,957,657; 3,957,658; 3,963,776; 4,038,195; 4,042,522; 4,049,556; 4,060,132; 4,060,489; 4,069,158; 4,090,967; 4,099,574; 4,149,599; 4,203,850; 4,209,407.
In U.S. Pat. No. 4,089,804, R. A. Falk discloses a method to improve R.sub.f -surfactants by employing water-insoluble R.sub.f -synergists, and in U.S. Pat. No. 4,090,967, AFFF agents are disclosed by Falk which contain R.sub.f -surfactants, R.sub.f -synergists and hydrocarbon surfactants among other ingredients. With the help of water insoluble R.sub.f -synergists it has now become not only possible to reduce the content of the costly R.sub.f -surfactants in an AFFF agent by up to 50% (and still meet U.S. military specifications for AFFF agents), but it also became possible to improve and utilize certain R.sub.f -surfactants in AFFF agent compositions which in the absence of the novel water-insoluble R.sub.f -synergists would not provide aqueous solutions with positive spreading coefficients.
However, a deficiency in prior-art AFFF agents continues to be (a) the amount of high priced fluorochemical required to achieve the proper performance, which limits the use of AFFF agents and (b) the inherent fish toxicity due to the high surfactant content in AFFF agents.
Since the AFFF agents ultimately enter the aquatic ecosystem, it has been customary to choose at least one or two aquatic species to assess potential aquatic toxicity and extrapolate generally two other species. The U.S. Navy screens AFFF agent toxicity on the Mummichog (fundulus heteroclitus) in artificial sea water. Agents are compared by 96 hour active exposure tests to determine the concentration lethal to 50% of the organisms; this is the 96 hour median lethal concentration for LC.sub.50.
It has now been found that the novel R.sub.f R.sub.f ion-pair complexes are partial or complete substitutes for the fluorochemical surfactants used in prior-art AFFF agents which will (a) reduce cost due to the use of smaller amounts of the more efficient ion-pair complexes, (b) will increase the efficiency of the AFFF agents and (c) reduce the fish toxicity significantly.